The buildup of static electrical charges on photographic films and papers is a problem of long standing in the photographic arts. These charges arise during the manufacture, handling and use of photographic recording materials and are affected by the conductivity and moisture conditions under which the material is handled. Accumulation of static charges is undesirable because it can cause irregular fog patterns in a photographic emulsion layer, an especially severe problem with high-speed emulsions. Additionally, static charges attract dirt to the photographic recording material and this can cause repellency spots, desensitization, fog and physical defects.
To overcome the adverse effects resulting from accumulation of static electrical charges, an antistatic layer is generally included in photographic materials. Typically, such antistatic layers are composed of materials which dissipate the electrical charge by providing a conducting surface. They must also adhere firmly to the film support, resist various chemicals such as processing solutions and they must not adversely affect the physical properties of the photographic element. Additionally, both the composition of antistatic layers and the method of their application to the photographic material substrate (backing) should be adapted for use with existing equipment in an existing plant. Use of hazardous chemicals should be avoided.
A large number of different materials have been proposed for use in antistatic layers of photographic products. For example, U.S. Pat. No. 2,527,267 describes an anti-stat layer comprised of an organic solvent soluble gelatin sulfonate complex; U.S. Pat. No. 3,791,831 describes an antistat layer composed of a mixture of sulfonated and carboxylated polymers; U.S. Pat. No. 4,582,782 discloses an anti-stat layer formed from a sulfonated polymer and an epoxysilane compound; U.S. Pat. No. 5,108,884 describes incorporation of a sulfonated polymer into the gelatin layer; and GB 1,496,027 describes an antistat layer which employs an acid form of a sulfonated polymer. Typically, the antistatic layers produced by conventional methods suffer from deleterious physical characteristics such as excessive swelling of layers incorporating the antistatic materials and poor adhesion of the antistatic layer to the support. Furthermore, diffusion of antistatic layer components into other layers diminishes clarity and antistatic performance. Many such antistatic layers are, additionally, not permanent as the antistatic components leach out on photographic processing. Finally, the antistatic layers are prepared by processes which generally require large volumes of volatile and toxic organic solvents, and/or expensive, volatile, toxic or corrosive organic reagents in generating the antistat layers.
In other approaches, crosslinking has played a role in an effort to improve antistatic protection. U.S. Pat. No. 4,147,550 describes internally crosslinked sulfonated microgel particles for dispersal in photographic layers. The crosslinking of antistatic polyelectrolytes, especially via aziridines, has been employed to improve the extent and permanence of the antistatic protection. The use of aziridine crosslinking agents is generally well-known. For example, U.S. Pat. No. 3,907,756 describes surface coating compositions prepared by reacting a polyfunctional aziridine with a carboxylic acid ester and U.S. Pat. No. 4,645,789 discloses the use of aziridines to crosslink carboxylate containing polymers. Relating directly to antistatic compositions, U.S. Pat. No. 4,225,665 discloses an antistatic layer containing a crosslinkable copolymer and a crosslinkable hydrophobic latex, and U.S. Pat. No. 4,859,570 describes a reaction product of a water-soluble crosslinkable electrically conductive copolymer and a polyfunctional aziridine as a crosslinking agent. The aziridine is said to interlink the copolymer via the carboxyl groups onto a support surface. U.S. Pat. No. 5,004,669 describes an antistatic layer containing a conductive copolymer having crosslinkable groups; and U.S. Pat. No. 5,079,136 describes an antistatic layer which utilizes a crosslinkable latex along with the conductive polymer. Antistatic layers derived from such compositions, however, suffer from poor wet adhesion characteristics and require high temperature curing steps, e.g., 100.degree. C. to 160.degree. C., to effect the crosslinking and to render the layer permanent.
Photographic materials provided with conventional antistatic layers, such as the ones described above, suffer from other significant disadvantages. For example, in certain instances the antistatic layer has inadequate antistatic properties, especially for high speed large grain emulsions, such as those used in phototypesetting papers. Inability of the antistatic layer to withstand photographic processing baths, which can involve temperatures of about 40.degree. C. and higher, and consequent leaching of the components of the antistatic layer into the processing baths and forming an undesirable sludge in the processing solution is also a serious problem. In general, conventional antistatic coating compositions often do not provide an effective antistatic layer which is durable, non-tacky, resistant to blocking and abrasion, strongly adherent to the support and inexpensive to use and to apply to a photographic support. Also, certain prior art antistatic coatings require high temperature cures, which makes their use incompatible with existing manufacturing equipment and processes capable of operating only at low temperatures.
Because of the aforementioned deficiencies in conventional antistatic layers, it would be desirable to provide a novel antistatic layer, permanent against photographic processing solutions, and having satisfactory antistatic properties but not requiring the charged polyelectrolyte to be crosslinkable and/or a high temperature cure.
Accordingly, it is an object of this invention to provide a permanent transparent antistatic layer for a polymeric photographic material support. As defined herein, the term "permanent" means that the antistatic layer remains conductive (charge-dissipative) throughout the normal use cycle of the photographic element. Specifically, the antistatic property shall be retained even following passage of the photographic element through processing baths and hot air drying.
Another object of the invention is to provide an aqueous-based permanent antistatic layer which does not require a high-temperature thermal curing step. That is, the antistatic layer is curable at temperatures ranging between about 40.degree. C. and about 50.degree. C.
These and other objects of the invention will become apparent in light of the detailed description and examples below.